The first related application, whose full contents are herein incorporated by reference, describes a magnetic structure designed for NMR head imaging in interventional procedures. The requirements to be satisfied are described in detail in the related application. These requirements are satisfied by a permanent magnetic structure capable of generating a uniform field for NMR imaging over a region of interest encompassing the head of a patient and that is open at one side allowing access to the head of the patient by a physician for surgical or other medical purposes. In a preferred embodiment described in that application, the magnetic structure of the invention comprises a head structure generating a uniform magnetic field in a region of interest encompassing the patient's head, coaxially aligned with a shoulder structure generating a magnetic field that extends the uniform field to a second larger region encompassing the patient's shoulder, and joined to the head structure by a coaxially aligned transition structure which maintains the uniform magnetic field in the region connecting the first and second regions. A feature of that invention is a magnetic structure comprising a single pole piece that extends throughout the structure, which pole piece is connected to the head, shoulder, and transition magnetic structures, with the single pole piece in the shoulder region being configured to house a second magnet that cooperates in extending the uniform field to the shoulder region.
The second related application, whose full contents are herein incorporated by reference, describes a magnetic structure designed for NMR head imaging in interventional procedures which differs from the first application in that the permanent magnetic system is based on the use of a conical magnet or section of a conical magnet. In a preferred embodiment, the conical section is connected to an inner essentially cylindrical section, and the region inside the conical magnet comprises a high-permability ferromagnetic material. In a further preferred embodiment, the magnetic structure comprises opposed, spaced circular-symmetric structures with the region of interest falling within a cavity between the circular-symmetric structures and a second larger outer cylindrical magnet extends over each of the circular-symmetric structures. The three main permanent magnetic components of the structure of the preferred embodiment are all magnetized in the same direction, preferably parallel to the axis of the conical segment.
The second and third referenced applications describe configurations of the sections bordering the outer curved surface of the inner pole piece in order to ensure that that outer curved surface is an equipotential surface, a condition necessary to optimize the field uniformity in the region of interest. In the second referenced application, it was believed necessary to provide a triangular non-magnetic region adjacent that outer curved surface, and in the third referenced application it was believed necessary to provide magnetic straps in certain desired positions to achieve the desired equipotential condition. Both of those solutions were less than optimal because of the structural complexities of the permanent magnetic members plus the possible reduction in field intensity in the critical region of interest.